1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) filter for exciting a surface acoustic wave on a substrate made of a piezoelectric material and for selectively passing a desired frequency band. More particularly, the present invention relates to a ladder-type SAW filter suitably used as an RF filter in mobile communication equipment.
2. Description of the Related Art
In recent years, SAW filters are increasingly in demand with the miniaturization of mobile communication equipment. The SAW filters are required to have superior characteristics of large stop band rejection as well as low loss. Among various types of SAW filters, ladder-type SAW filters are promising ones because the loss thereof can advantageously be made lower as compared with interdigitated interdigital transducer structure type SAW filters in which a plurality of input interdigital transducers (IDTs) and output IDTs are alternately disposed in a direction in which the surface acoustic wave propagates.
Conventional ladder-type SAW filters are described, for example, in Japanese Laid-Open Patent Publication No. 52-19044 or Japanese Laid-Open Patent Publication No. 5-183380.
Japanese Laid-Open Patent Publication No. 52-19044 is directed to a ladder-type SAW filter for realizing narrow-band filter characteristics of sufficiently narrow pass bandwidth and sufficiently large stop band rejection. For this purpose, the ladder-type SAW filter of the above publication has a construction in which a series branch and a parallel branch, each having an SAW resonator, are provided on a substrate made of a piezoelectric material (hereinafter referred to as a piezoelectric substrate). In addition, the ladder-type SAW filter is constructed so that the resonance frequency of the series branch resonator is made equal to the antiresonance frequency of the parallel branch resonator, so that an equal frequency is set to be the center frequency of the pass band. The narrow-band filter characteristics of sufficiently narrow pass bandwidth and sufficiently large stop band rejection are realized by setting the equivalent parallel capacitance of the parallel branch resonator larger than the equivalent parallel capacitance of the series branch resonator, and hence making the ratio of equivalent parallel capacitance larger than 1.
In the above-described construction for realizing the narrow-band filter, the stop band rejection can be made larger by increasing the ratio of equivalent parallel capacitance, while the bandwidth becomes narrower as the ratio of equivalent parallel capacitance is increased.
In the ladder-type SAW filter described in Japanese Laid-Open Patent Publication No. 5-183380, an inductance is additionally connected to the parallel branch resonator, so that the antiresonance frequency of the parallel branch resonator is shifted to the lower side. Accordingly, the bandwidth as one of the filter characteristics can be controlled by using a difference in frequency between the resonance frequency and the antiresonance frequency of the parallel branch resonator. Thus, the additional provision of the inductance makes it possible to widen the bandwidth, by increasing the difference in frequency between the resonance frequency and the antiresonance frequency.
In the above-described construction, in order to ensure a large stop band rejection, it is necessary to increase the ratio of the equivalent parallel capacitance or to increase the number of serially-connected ladder-type filters. However, since an inductance is additionally connected to the parallel branch resonator in series as described above, the electric resistance and inductance of the connecting wiring between SAW resonators on the piezoelectric substrate and wire bonding considerably affect the characteristics of the parallel branch resonator. As a result, the resonance frequency of the parallel branch resonator is also shifted to the lower side. Hence, a steep rise to the pass band cannot be obtained in the filter characteristics, and consequently, a satisfactory stop band rejection cannot be attained.
As described above, the conventional ladder-type SAW filters involve a difficulty in obtaining both a wide bandwidth and a sufficient stop band rejection.